The first thing to do is to turn it off and then back on to reset it and see what happens. If it still fails to light, it is time to check the ignitor and it's operation, assuming that it was working previously. If you are just turning it on for the first time, afther having had the propane turned off for an extended period, you might have air in your lines. If that is the case, start by lighting one of your stove burners and let it burn for a few minutes to be sure that all air is vented. Once you do that it still might take several cycles before it lights.
If it was working, take a close look at the ignitor. Make sure that the water heater is turned off and then remove the igniter assembly. Clean it with some Scotch Brite or similar product. Check the gap to make sure that it is 1/4", +/- 1/16". When you put it back in, make sure that the igniter is directly in line with the path of the gas flow.
Next, have someone turn the water heater on while you watch. Watch for a spark from the igniter. It should spark at least three times. When that happens, does the gas flare at all or attempt to light? If it does light, but then goes back out, is the igniter in the flame so that it gets heated by the flame? It has to be directly in the flame to send a signal back to the circuit board to let it know that the gas did light. If it lights but then turns back off when the ignitor stops sparking, that would mean that the heat was not detected. If it does not light at all, get one of the long butane lighters like they sell to light a gas barbecue and insert it into the area where the gas jet is located. Light it and have your helper trun the heater on again. If the gas flows at all, you will see some flairing of it, or if flowing properly it should light. If there is no gas, the gas valve is very likely the problem. To check that, use a meter when the water heater is first turned on there should be 12V-dc applied to the two terminals of the valve. If the gas does light from the butane lighter, it is either the igniter or the circuit board. I would replace the igniter first as it is not very expensive while the circuit borad is expensive and the igniters are the most likely to fail.
LP on, leak detector on, no cob webs in tube, water in tank, LP tank full, ..... Kirk has good points to check. Might be solenoid stuck (might tap it on top lightly). Of course, if lighter don't work, it is not getting gas. Mine is electronic and will click when trying to ignite. You can hear it quite clearly. Just some thoughts
$120.00 is too much. If you are no longer under warranty go to www.rvmobile.com and order a dinosaur UIB-S and cover. It is an aftermarket ignition board thats better than OEM and comes with a 3 year warranty.